The ambush occurred in the southern Saydia neighbourhood where a truck bomb exploded yesterday, killing at least 10 people and injuring more than 60 outside a police station.

The assailants, driving an unmarked white car, made a hasty getaway after gunning down Jassem Kadhem, the spokesman said.

Mr Kadhem, one of about 25 director generals at the newly re-established Defence Ministry, was the latest in a long line of high-profile figures targeted in a 15-month insurgency that bred during the US-led occupation.

On Saturday, Justice Minister Malek Dohan al-Hasan, 83, emerged unscathed after a suicide car bomber hit his motorcade in the Iraqi capital but three of his guards, including a nephew, were killed along with two civilians.

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Al-Qaeda-linked militant Abu Mussab al-Zarqawi reportedly claimed responsibility for the shooting attack. The Jordanian-born Zarqawi has also offered a $US285,000 ($A390,545) reward for the death of the Iraqi Prime Minister, Iyad Allawi.

The Prime Minister's own offices and residence were targeted by mortar fire in early July. The shells missed their target but injured five people nearby.

Insurgents have waged an assassination campaign against police, civil servants and politicians in a bid to derail the new government and the country's US-led reconstruction efforts.

US Army Lieutenant-Colonel Bill Salter said between 10 and 15 people were killed and more than 60 injured in the truck bomb attack.

By Saturday, 36 US soldiers had died this month, compared with 42 last month.

The attack came as figures were released showing that almost as many US soldiers lost their lives in Iraq in the first half of this month as in all of the previous month, even as Iraqi insurgents seem to have shifted their focus from attacking US targets to striking Iraqi ones.

Since the June 28 handover of power, the 160,000 coalition forces have averaged more than two deaths a day, among the highest rate of losses since the war began 15 months ago. By Saturday, 36 US soldiers had died this month, compared with 42 last month.

Amid the violence, Prime Minister Iyad Allawi on Sunday ordered the reopening of a radical Shiite newspaper. The closure of the paper in March had triggered some of the worst anti-American violence of the occupation.